Laser radar devices are useful in underwater imaging and in the identification of high speed moving objects such as missiles or airplanes. Laser radar has the capability to produce 3-dimensional images of objects and to measure with high precision the range and angular location of an object.
FIG. 1 shows a schematic of a laser radar device for creating an image of an object 5. The laser radar device has a pulsed laser 1, a photodetector array 6 with imaging lens assembly 2, a signal processor 3, and a computer 4. The laser 1 is synchronized with the signal processor and computer. In operation, the laser 1 produces a short duration (e.g. less than 5 ns) light pulse 7 directed at the object 5. Backscattered light from the object is imaged upon the photodetector array 6. The image on the array provides 2-dimensional (angle-angle) information about the object in the plane of the photodetector array 6. The arrival time of photons at the array provides depth and range information about the object (i.e. information in the third dimension, perpendicular to the plane of the photodetector array 6). Typically, the arrival time of the photons is divided into a number of time bins corresponding to different ranges. Therefore, the laser radar device provides three dimensional information about the object. In the prior art laser radar system of FIG. 1, backscattered light from a single light pulse 7 is imaged upon the entire array 6. Thus there is no need for a mechanical two-dimensional scanner as needed for other prior art laser radar systems. The 3-dimensional information from all photodetector or pixels in the array is output to the computer which calculates an image. The computer may also calculate certain structural or optical characteristics of the object. Also, the computer may identify the object based on its three dimensional shape.
For accurate range information, it is essential that the laser and signal processor be accurately synchronized so that the time-of-flight of the photons is accurately measured (i.e. within several nanoseconds). Also, it is important for the photodetector array to have a fast response time. Arrays such as PIN diode arrays or multi-anode photomultiplier tubes can be used, for example. Each photodetector in array 40 simultaneously receives light from region 43, and each photodetector has its own circuit 20.
In laser radar applications such as object identification and imaging, it is important to acquire intensity information as a function of voxel (volume element) in the 3-dimensional image. In other words, each voxel must have an associated intensity measurement.
Some imaging radar systems only measure range to a first target surface for each pixel. More information is available for imaging if each pixel provides information over a distribution of range values.
One conventional approach to measuring optical intensity in each voxel is to connect each pixel in the photodetector array 6 with a high speed analog to digital converter. Each A-D converter provides a digital representation of the intensity in each voxel. A disadvantage with this approach is that one A-D converter is required for each pixel. This increases the size, weight and cost of the device. For precision ranging, very high speed A-D converters are required, performing a measurement every few nanoseconds. Finally, a high data transfer rate to the computer is required. Another problem with this approach is that, for high intensity resolution and high dynamic range intensity measurement, the A/D converter must approximate too many bits of precision. It is difficult to provide many bits of precision for each photodetector in the short time scales necessary.
The conventional approach to signal conversion is to use a sample/hold circuit in conjunction with an A/D converter. The signal from a photodetector must be constant for an `aperture time` while the A/D conversion is performed. The analog signal may change during the aperture time, leading to errors in the digital representation produced. Sample/hold A/D converters have relatively long aperture times compared to aperture times preferred for high resolution laser radar imaging.
Another approach to measuring voxel intensities is to use a number of CCD arrays, with each array gated with an associated micro-channel plate image itensifier tube. The CCD arrays are turned on at slightly different times, therefore providing range information. A problem with this approach is that it requires complex signal processing to integrate the signals from the different CCD arrays. Also, multiple imaging arrays and intensifier tubes increase the weight and cost of the device.
U.S. Pat. No. 5,446,529 to Stettner et al. discloses a method for obtaining intensity information for each voxel in a 3-D laser radar image. Stettner employs an array of charge storage capacitors for each pixel in the photodetector array. The capacitors are sequentially connected to their associated pixel during data acquisition so that a single capacitor is associated with each voxel in the scene being imaged. MOSFET switches are used to connect the capacitors with the pixels. Each capacitor stores a charge related to the light intensity from a voxel. The charges on each capacitor are then converted into a digital representation which is processed by a computer. The approach of Stettner et al. necessarily has a `dead time` while switching capacitors. The dead time limits the time resolution, and therefore the range resolution.
Another problem with laser radar generally is that the backscattered light may have a wide range of intensities spanning several orders of magnitude. Each pixel in the photodetector array may receive only a single photon, or many thousands of photons. Straightforward processing of the photodetector signals requires a receiver (i.e. an A-D converter) with a large dynamic range. A large dynamic range receiver is either expensive, or provides limited resolution at very high or very low light intensities.